Obtaining an accurate three-dimensional profile of a surface for measurement purposes and checking integrity of objects, or parts produced through a process, has been subject to ongoing research and development. Need for quickly measuring size of objects, particularly in three dimensions, stems from stringent requirements for product quality, process control and process monitoring. These needs are driven by increasingly higher manufacturing rates of mass produced goods and continuing miniaturization of components. For example, with respect to electronics manufacturing, requirements for checking size and position of components is extremely critical. Here, a solder paste material used to bond components to printed circuit boards must have a known volumetric size to ensure that electrical and mechanical characteristics of a solder joint meets quality requirements of the product.
Non-contact, three-dimensional measurement techniques are generally preferred over tactile methods because non-contact techniques can be used to inspect electronic components and other delicate parts without touching, or possibly destroying the components or parts to be measured. Moreover, due to high manufacturing rates, speed of measurement is an important feature. For this reason, non-contact, optical techniques have received more acceptance as they are generally faster than other measuring techniques. Among many different optical methods that have been developed, two approaches have become more commonly used than others. A first class of these techniques generally includes triangulation [1-7], and a second class generally includes fringe-based techniques (through Moiré or phase-shifting profilometry) [8-2]. Triangulation is a relatively effective technique wherein a beam or sheet of light, often a laser source, is projected obliquely onto a surface being examined and a deformed image of a reflected spot or line is generated and captured using a camera, such as a CCD camera. Deformations of the spot or line are analyzed by using a computer to derive height information. Effectiveness of triangulation is limited to those relatively few applications wherein a reflection from surfaces of objects to be measured is of sufficient intensity for a camera to register an image, but not so intense that over-exposure or “blooming” of the image occurs. In practice, finding a reasonable compromise of lighting is often difficult, if not an impossible task. This is particularly true in cases where a source of illumination is a laser. For example, electronic parts and printed circuit board assemblies contain objects of widely different reflectivity, such as copper traces, solder paste, metallic objects, solder mask, screen print, plastic-encased components, and other such objects. As such, finding an imaging illumination that will work for all types of reflectivities associated with these components and substances is difficult, and in some instances impossible. Moreover, resolution of triangulation-based systems is primarily proportional to magnification, which in turn depends to some extent on pixel resolution of the camera. To obtain high accuracy, magnification needs to be as high as possible, which in turn demands high pixel resolution. Pixel resolution in turn determines throughput or sampling rate.
Fringe-based methods derive depth information through manipulation of image intensities obtained from a phase-shifted source of illumination. These methods offer certain advantages over triangulation, such as higher resolution and ability to perform full-field three-dimensional imaging. A main disadvantage of these methods is complexity involved in processing interfrograms or phase-shifted images, such as unwrapping of phase images. Moreover, similar to triangulation, these techniques also require that imaging conditions, particularly illumination intensity, be carefully controlled to avoid underexposure and overexposure of components to be measured. As mentioned above, in complex scenes containing objects of widely differing reflectivity, this is usually impractical.
In contrast to the above-mentioned methods, three-dimensional measurement techniques that use only a reflection of structured lighting have been left generally unexplored, perhaps because of a lack of understanding that intensity variations due to surface reflectivity changes need to be decoupled from intensity variations caused as a result of changes in a surface profile.
One method of relating surface height variations to intensity modulations has been presented in [13] wherein two sources of structured light are used to measure height of components and objects. The two sources of light are arranged such that objects of different heights illuminated by structured light will reflect different levels of light. As a result, intensity of reflected structured light is proportional to height of a surface that reflects the light. However, this method has three main drawbacks, a first of which requiring that the two light sources be identical in output, in turn requiring calibration of the light sources. Second, obtaining a surface profile of objects requires two passes. With each pass, one of the light sources is used to capture an image of the surface. Since the scanning process is a mechanical operation, speed of scanning will certainly be a limiting factor for many applications. Third, the technique suffers from the common problem of underexposed or overexposed images of components due to different surface reflectivity of these components. Of course, it is possible to take numerous passes over a surface, each pass being at a different illumination setting, but this would be impractical for high-speed applications.